Flexographic, lithographic, intaglio, gravure and other processes, including those in which a central impression cylinder is used, for printing a wide variety of articles are known in the art. Recently, these and other processes and techniques have been used to print very small, fine features, particularly for forming electrical circuits and circuit components.
Today, the intaglio printing process is oftentimes used for printing very small features. Typically, the intaglio process uses a metal plate that has features engraved into its surface. Often, the engraving is done by hand. Other methods of forming features on the metal plate can also be used as well. The plate can be used in a generally planar state, or, if the plate is thin enough, it can be wrapped around a curved object, such as a cylinder, and used in a high speed printing press.
During the intaglio printing process, the plate is typically inked with a high viscosity, paste-style of ink. The plate is then wiped to remove the excess ink from the top surface, leaving ink only in the recessed areas. A substrate, such as paper, film, and the like, is then brought in contact with the inked plate. A very high pressure is used to transfer the ink from the plate to the substrate. However, the high pressure often results in a deformation of the substrate.
For printed electronics applications, it is desired to print features that are about 10 microns wide or less, and even sub-micron sized features. While the intaglio process is able to print fine features of about 20 microns or less, there are some drawbacks that render traditional intaglio printing for printed electronics applications undesirable.
One drawback of the intaglio process is the fabrication of the plates. Engraving is a slow process, and it can only by done one plate at a time. If done by hand, there is a high rate of error.
Another drawback of the intaglio process is the distortion of the substrate which results in a high degree of surface topology and stress in the substrate. Increased surface topology can negatively impact electronic device performance by reducing printed element quality due to less effective ink transfer on a rough surface for subsequent printed layers. In addition, the distortion of the substrate can create discontinuities in previously printed features. Stress in the substrate can be relaxed in subsequent process steps, such as annealing, leading to changes in dimensions which can also negatively impact electronic device performance because of misalignment between layers, changes in critical dimensions such as the channel length and width of a transistor, and imprecise registration. Another drawback of the intaglio process is the use of a paste ink. For electronic applications, it is desired to print with materials that are relatively pure, such as, for example, a semiconductor or a metal. A paste ink requires the addition of rheology control materials that reduce the purity of the final deposited material, which can negatively impact electronic device performance by reducing the conductivity of a metal or mobility of carriers in a semiconductor material.
Another process that can be used for printing fine features is gravure printing. Gravure printing is generally considered a sub-set of the intaglio process. In gravure, a cylinder is engraved with cells, rather than continuous lines of intaglio printing. Engraving is done with a stylus or a laser. The width, length, and depth of the cell can be controlled. Ink is applied to the cylinder to fill the cells, and the excess is subsequently removed with a doctor blade. Typically, the ink has a much lower viscosity when compared to the intaglio printing process.
A substrate is then brought in contact with the cylinder and pressure is used to assist a transfer of the ink to the substrate. The pressure is lower, however, compared to intaglio printing so there is minimal to no distortion of the substrate. However, gravure printing does have a number of drawbacks, including the minimum feature size that can currently be printed. As stated above, for printed electronics applications, it is desired to print features that are about 10 microns wide or less, and even sub-micron sized features. Today, gravure printing can print a minimum feature size of only about 20 microns.
Another drawback is that the use of the cells in gravure printing results in wavy edge lines, rather than the desired straight lines. Also, the use of cells can create a non-uniform thickness of the deposited ink. The non-uniform thickness of ink can result in discontinuous lines that prevent electrical current flow.
Further, gravure printing cylinder fabrication is cost-restrictive. It is also more difficult to image a gravure cylinder compared to a flat surface due to the curved nature of the cylinder. It can be time and/or cost restrictive, and involves a high risk of error.
Other known additive printing processes, such as flexographic and ink-jet, suffer from several drawbacks as related to the printing of transistors and other small-scale electrical circuits and devices. One drawback relates to a registration and resolution of a printed electrical circuit, such as a transistor comprising multiple individually printed layers. Flexography, for instance, has been reported to print features only as small as about 50 microns and typical flexographic presses have registration control no better than about 40 microns from one printed layer to the next. Ink jet techniques suffer from slow speed and low reliability. Because each layer interacts with other layers to conduct or insulate, elevated printing quality is necessary to form layers that properly interconnect to form an operable electrical device. Another drawback relates to higher costs presently associated with printing electrical circuit devices that meet quality standards. In view of these and other drawbacks, current printing processes are not capable of achieving and maintaining quality standards at micron-measured levels without sacrificing performance, cost, and efficiency.
Currently, electronic devices are commonly created by a photolithography and etching process. Photolithography processes typically used in the silicon-based semiconductor industry are photographic processes used to transfer circuit patterns onto a semiconductor wafer. This is done by projecting light through a patterned reticle, or a glass plate with a layer of chrome or other masking agent on one side, onto a silicon wafer covered with a photosensitive material (photoresist). The exposed portions of the circuit are then wet or dry etched to pattern the circuit. The photolithography process is utilized in the fabrication of electronic devices and circuits because of the ability to etch fine features, such as on the order of 10 microns or less. However, these processes—deposition, photolithography, and etching—are repeated many times in the formation of traditional semiconductor devices thereby creating a large amount of waste, and can be time intensive relative to additive printing processes. Furthermore, because of the time involved and the amount of waste, these processes can be costly compared to additive printing processes.
There remains a need for patterned printing plates, printing systems, and methodologies that are capable of producing high quality electrical circuit products with high resolution fine features, with minimal substrate distortion, precise registration, and at a reduced per-unit cost.